(1) Field of the Invention
This invention relates to a method of processing a substrate made of a ferroelectric single crystalline material, and more particularly to a method of processing a substrate of a ferroelectric single crystalline material suitable for a Second-Harmonic-Generation device using the Quasi-Phase-Matched type, an optical modulator, an optical switch and the like. This invention also relates to the substrate as well as an optical waveguide element using such a substrate.
(2) Related Art Statement
The Second-Harmonic-Generation (SHG) device using the Quasi-Phase-Matched (QPM) type, in which a periodical polarizing inversion structure is formed on a LiNbO3 or LiTaO3 single crystal, is expected as a light source for a blue laser usable as an optical pickup or the like. This device can be widely used as an optical pickup or the like for optical disk memory, medicine, photochemistry, and various optical measurements.
An optical modulated device such as an optical modulator using an optical waveguide formed on a LiNbO3 single crystal or an optical switch, is expected in the field of the optical communication or the information processing.
The optical waveguide device used as the SHG device, the optical modulator or the like is generally manufactured as follows.
A substrate of a ferroelectric crystalline material such as LiNbO3 is proton-exchanged by dipping it in a solution of benzoic acid or a solution of pyrophosphoric acid to form a desired proton-exchanged pattern, and diffusing the protons into the substrate by annealing it at 300 to 400xc2x0 C. for 0.1 to 5 hours, whereby an optical waveguide is formed.
In another method, an optical waveguide is formed by vapor depositing a metal such as Ti onto a substrate of a ferroelectric single crystalline material such as LiNbO3 to form a pattern, and thermally diffusing the Ti metal or the like into the substrate under heating at 900 to 1100xc2x0 C. for 4 to 40 hours.
However, in these methods, it has been difficult to form an optical waveguide having a stepped refractive index distribution with a large optically confining effect, so that it has been difficult to attain sufficiently large optical fiber coupling efficiency (the amount ratio of output light to input light in an optical waveguide device). Moreover, light damaging resistance or optoelectrical constant unfavorably was deteriorated.
Considering the above problems, NGK Insulators, Ltd. invented a method of manufacturing a new optical waveguide device suitable for use in a SHG device or the like, and filed Japanese patent application No. 9-52,679 directed to this new optical waveguide device-manufacturing method.
That is, a substrate of ferroelectric single crystalline material such as LiNbO3 is proton-exchanged by dipping it in a solution of benzoic acid or a solution of pyrophosphoric acid, a proton-exchanged portion is selectively removed by wet-etching the thus obtained proton-exchanged layer with a solution of hydrofluoric acid or a solution of nitric acid to thereby form a recessed portion having a substantially semicircular sectional shape, and thereafter a ferroelectric optical waveguide is formed, through a film formation, by a liquid phase epitaxial method, on the substrate in which the recessed portion is formed.
However, the above method being used, when the temperature of proton-exchanging is higher than 180xc2x0 C., a half opening width xe2x80x9crxe2x80x9d in the recessed portion formed in the ferroelectric substrate is larger than the depth xe2x80x9chxe2x80x9d therein as shown in FIG. 1, whereby the recessed portion often has a shape having semi-elliptical cross section which is shallow in the vertical direction to the substrate.
In this specification, as apparent from FIG. 1, the wording xe2x80x9chalf opening widthxe2x80x9d is defined as a half value of opening width in the recessed portion.
It is an object of this invention to provide a method of processing a substrate of ferroelectric single crystalline material such that a concave ditch structure having a recessed portion with a depth equal to or larger than an half opening width, or a convex ridge structure having shoulder portions with a height equal to or larger than a radius curvature of each of curved portions and that the concave ditch structure and the convex ridge structure can be used for forming onto the substrate an optical waveguide exhibiting a stepped refractive index distribution which has large optically confining effect.
This invention relates to a method of processing a substrate made of a ferroelectric single crystalline material, comprising the steps of forming a desired proton-exchanged layer in the substrate by proton-exchanging a portion of the substrate, and selectively removing the proton-exchanged layer to form a concave ditch structure in the ferroelectric single crystalline substrate, wherein the desired proton-exchange layer is formed by using an acid containing a lithium salt as a proton-exchanging source, the surface of the substrate from which the concave ditch structure is formed is an X-cut surface or a Z-cut surface, as a main surface, of the ferroelectric single crystalline material used as the substrate, and the concave ditch structure has a recessed portion with its depth equal to or larger than its half opening width.
This invention also relates to a method of processing a substrate made of a ferroelectric single crystalline material, comprising the steps of forming a desired proton-exchanged layer in the substrate by proton- exchanging portions of the substrate, and selectively removing the proton-exchanged layer to form a convex ridge structure, wherein the desired proton-exchange layer is formed by using an acid containing a lithium salt as a proton-exchanging source, the surface of the substrate from which the convex ridge structure is formed is an X-cut surface or a Z-cut surface, as a main surface, of the ferroelectric single crystalline material used as the substrate, and the convex ridge structure has shoulder portions with their height equal to or larger than a radius of curvature of each of curved portions.
According to the method of processing a substrate of the ferroelectric single crystalline material of this invention, as shown in FIGS. 2(a) and 2(b), a concave ditch structure having a recessed portion, in which its half opening width xe2x80x9crxe2x80x9d is equal to (FIG. 2(a)) or less than (FIG. 2(b)) its depth xe2x80x9chxe2x80x9d, can be formed on the substrate of ferroelectric single crystalline material, or a convex ridge structure having shoulder portions, in which its radius of curvature xe2x80x9cRxe2x80x9d of each of curved portions 6 is equal to (FIG. 3(a)) or less than (FIG. 3(b)) its height xe2x80x9cHxe2x80x9d, can be formed on the substrate of ferroelectric single crystalline material.
Although these reasons are not apparent, it is considered as follows.
Generally in a ferroelectric crystal such as LiNbO3, a diffusion coefficient of protons therein changes depending on a dissociation constant of an acid used for proton-exchanging and its crystal orientation.
Generally as the dissociation constant of the acid is large, the proton diffusion coefficient is large, and as the dissociation constant thereof is small, the diffusion coefficient thereof is small.
Moreover in the ferroelectric crystalline material, the proton diffusion coefficient in the Z direction of the crystal is smaller than in the X or Y direction.
According to this invention, when an acid containing a lithium salt as a proton source is used, the dissociation constant of the acid can be made smaller to thereby be capable of controlling the dissociation constant. Moreover, when an X-cut surface or a Y-cut surface of a ferroelectric crystal is used as a main surface of a substrate of ferroelectric single crystalline material, the dependency between the dissociation constant of the acid and the crystal orientation in the diffusion coefficient of protons can be adjusted in a balanced manner, whereby the proton diffusion coefficient in the vertical direction to the main surface of the substrate can be equalized to or made larger than that in the parallel direction to the main surface of the substrate.
Therefore, a proton-exchanged layer in which protons diffuse equally in the vertical direction and the parallel direction to the substrate can be obtained. Alternatively, a proton-exchanged layer in which protons diffuse in the vertical direction more greatly than in the parallel direction to the substrate can be obtained. By selectively removing the proton-exchanged layer, as shown in FIGS. 2(a) and 2(b), the concave ditch structure having a recessed portion with its half opening width xe2x80x9crxe2x80x9d being equal to or less than its depth xe2x80x9chxe2x80x9d is formed, or as shown in FIGS. 3(a) and 3(b), the convex ridge structure having shoulder portions with its height xe2x80x9cHxe2x80x9d being equal to or larger than the radius of curvature xe2x80x9cRxe2x80x9d of each of the curved portions 6, can be formed.
On the other hand, xe2x80x9cWet-Etched Ridge Waveguides in Y-Cut Lithium Niobatexe2x80x9d (J. LIGHTWAVE TECHNOLOGY, VOL. 15, NO. 10, OCTOBER, 1997, P1880xcx9c1887) shows that an acid containing a lithium salt is used to proton-exchange a Y-cut surface of a lithium niobate single crystal as a main surface of a substrate of ferroelectric single crystalline material.
This invention, however, requires that an X-cut surface or a Z-cut surface of ferroelectric crystal is used as the main surface of the substrate of ferroelectric single crystalline material. On the contrary, when a Y-cut surface of a ferroelectric crystal is used as the main surface of the substrate, as shown in FIG. 15 of this literature, only a convex ridge structure having a semi-elliptical sectional shape which is shallow in the vertical direction to the substrate surface is formed. Accordingly the object of this invention can not be attained.